var str = 'abc'//new String('abc')
str+=1  //str = {'abc'}+1 -> str = 'abc1'
var test = typeof(str) //
if(test.length === 6 ){
    test.sign = 'typeOf的返回结果可能是String' // new String(test).sign = 'XXXX'
    //delete test.sign 
}
console.log(test.sign); // new String(test).sign 